Reverse osmosis (RO) membranes are often used in large quantities to desalinate water on a commercial scale. Because many membranes are typically connected in parallel to a common output, it is hard to localize a performance degradation and/or failure to individual membranes. Therefore, to maintain output water quality at an acceptable level, membranes are typically replaced according to a time-based replacement schedule. A time-based replacement policy is undesirable for a variety of reasons:                The frequency of membrane replacement is not necessarily optimal. Under certain conditions, the membranes may be replaced too frequently, thereby reducing cost-effectiveness. Under other conditions, the membranes may not be replaced frequently enough, thereby reducing water output quality.        Not all membranes necessarily degrade at the same rate. Two membranes installed at the same time do not necessarily have to be replaced at the same time.        A membrane not only degrades in time, but is also subject to random failure that can result in the leakage of saline water into the output stream. Random failures of this sort cannot be effectively handled according to a time-based replacement schedule.        
It would therefore be beneficial to have a means of detecting the real-time in situ condition of RO membranes on an individual basis, so that the membranes can be replaced on an as-needed basis in response to performance degradation and/or failure. This goal is met by the present invention.